单调栈

http://poj.org/problem?id=2559
 1 a[n + 1] = p = 0;
for (int i = 1; i <= n + 1; i++) 
{
    if (a[i] > s[p]) 
    {
        s[++p] = a[i], w[p] = 1;
    } 
    else 
    {
        int width=0;
        while (s[p] > a[i]) 
        {
            width += w[p];
            ans = max(ans, (long long)width * s[p]);
            p--;
        }
        s[++p] = a[i], w[p] = width + 1;
    }
}

 及时排除不可能的选项，保持决策集合的高度和秩序性